The Internet of Things (IoT) is a vision for the future world where everything that can benefit from a connection will be connected. Cellular technologies are being developed or evolved to play an indispensable role in the IoT world, particularly the machine type communication (MTC). MTC tends to place lower demands on the network with respect to data rates, for example, as compared to mobile broadband. In general, MTC may be characterized as requiring low cost device design, better coverage, and the ability to operate for years on batteries without charging or replacing the batteries. To meet the IoT design objectives, 3GPP is currently studying the evolutions of existing 2G/3G/4G LTE technologies, including the study item “Cellular System Support for Ultra Low Complexity and Low Throughput Internet of Things” approved in GERAN#62. The current studies under GERAN include both GSM evolution and completely new designs. There are two main so-called “Clean Slate” approaches: (i) Narrowband FDMA, and (ii) Narrowband OFDMA. These Clean Slate approaches are narrowband (NB) systems with a carrier bandwidth of 200 kHz and target improved coverage compared to today's GSM systems, long battery life, and low complexity communication design. One intention with these approaches is to deploy it in spectrum that is currently used for GSM, by reducing the bandwidth used by GSM and deploying NB Clean Slate systems in the spectrum that becomes available. Another intention is to reuse existing GSM sites for the deployment of NB Clean Slate systems.
Cell search is an essential step for the proper operation of devices within a network. Successful cell search is needed for devices to receive and decode system information required for proper communication within a cell. During the cell search, devices acquire synchronization information including both symbol and frequency synchronizations of a cell. In addition, in the presence of multiple cells, the mobile station also needs to distinguish the particular cell on the basis of a cell ID, and obtain the corresponding frame number to perform frame synchronization. Thus, a typical cell search procedure consists of determining the timing alignment, correcting the frequency offset and obtaining both the correct cell ID as well as the frame ID.
When the device wakes up from deep sleep, for example from being in a power saving state, the frequency offset is to a large extent due to device clock inaccuracy, often assumed to be up to 20 ppm. The clock inaccuracy results in a drift in the timing of the sampling of the received signal. To the device receiver, this drift appears mainly as a frequency offset of the received signal, a continuous rotation of the received samples. For a system operating with 20 ppm with a carrier frequency of 900 MHz, the maximum frequency offset is 18 kHz. This offset needs to be estimated and corrected for.
Existing cell search approaches for Clean Slate systems are described as follows.
(I) Cell Search in NB FDMA—Option A
Cell search is assumed to be performed using three sequences:
(a) Primary Synchronization Sequence (PSS): The PSS is used to determine the frame timing alignment, along with a coarse estimation of the frequency offset.
(b) Secondary Synchronization Sequence (SSS): The SSS is used to obtain a finer estimate of the frequency offset. Together with the PSS, the SSS also determines the cell ID.
(c) Frame Index Indication Signal (FIIS): The FIIS is used to determine the frame number, i.e., the ID of the current frame in the superframe. Each superframe consists of 64 consecutive frames.
Every frame consists of 960 symbols. 256 symbols are dedicated to PSS, 257 for SSS, 127 for FIIS and the remaining 320 symbols are for carrying the broadcast information in a Broadcast Information Block (BIB).
An MTC device first needs to search for a signal in a viable frequency band after switching on. Signal detection is performed on the basis of comparing the amplitude of the peak from a correlation based detector with a pre-determined threshold. This is achieved by correlating the received signal with a known sequence, or a set of known sequences.
The combination of PSS and SSS is also used to determine the ID of the particular cell, after the MTC device has performed the timing and frequency synchronization. In order to achieve this functionality, three pre-defined sequences are used for PSS, and 12 are used for SSS, giving a total of 36 possible combinations. Each combination is used by a particular cell, and this in turn, enables the MTC device to determine the cell ID. Specifically, the MTC device first tests each of the three PSS to determine the one with the highest correlator output. This gives the frame timing, and then, the device tests each of the 12 SSS to determine the one with the highest output at the correlator to correct the frequency offset. Once the two sequences have been found, they correspond to one of the 36 possible combinations, which determine the cell ID. The next sequence, FIIS, is then used to obtain the frame number and this completed the cell synchronization procedure.
(II) Cell Search in NB FDMA—Option B
In this design for the cell synchronization procedure, three sequences are used. They are given as follows:
(a) Synchronization Sequence (SS): A single sequence is used for both the frame timing estimation and frequency offset correction.
(b) Cell ID Sequence (CIS): A separate sequence is used to determine the cell ID.
(c) Frame Index Indication Sequence (FIIS): A third sequence is reserved to determine the frame number.
The SS sequence is composed of 410 symbols, the CIS has 101 symbols, and the FIIS has 127 symbols. The remaining 322 symbols are used for carrying the broadcast information, making the total number of symbols in the frame equal to 960.
The cell search procedures are as follows. First, the symbol synchronization is achieved by a correlation-based search for SS. The SS is further used for frequency offset correction. After acquiring symbol and frequency synchronizations using the SS, the transmitted CIS sequence is detected by searching over the 100 CIS candidates. The detected CIS provides information on the cell ID. Lastly, the FIIS is then used to obtain the frame number and this completed the cell synchronization procedure.
(III) Cell Search in NB OFDMA
A downlink frame consists of 163 normal slots and 8 special slots. A normal slot consists of 14 OFDM symbols, each having 72 subcarriers. The OFDM sampling rate is 320 kHz with a FFT length 128. The 8 special slots are used for physical synchronization channels (PSCH), the transmission of which is of single carrier rather than OFDM. The symbol rate in a special slot is 160 kHz, allowing a rolloff factor 0.25 in raised cosine filter to be used in the 200 kHz channel. To match the 320 kHz OFDM sampling rate, the symbols in the special slots are 2× up sampled. The PSCH consists of two parts: PSS and SSS. The functions of PSS and SSS are as follows.
(a) PSS: A single sequence for initial symbol-level synchronization and carrier frequency offset (CFO) estimation.
(b) SSS: A SSS consists of two sequences, SSS-1 and SSS-2, conveying both the index of the associated special slot within a frame and cell-specific ID information. Further, SSS can be used to refine CFO estimation and to detect false alarm.
The PSS consists of 416 symbols and is based on a length-255 Kasami sequence. The SSS consists of 284 symbols and is generated based on length-71 Zadoff-Chu (ZC) sequences. In particular, both SSS-1 and SSS-2 are based on a length-71 ZC sequences.
The cell search procedures go as follows. First, the symbol synchronization is achieved by a correlation-based search for PSS. After acquiring symbol synchronization, the CFO is estimated using the PSS symbols. After acquiring symbol and frequency synchronizations using the PSS, the transmitted SSS sequence is detected by searching over the 70×70 SSS candidates. The detected SSS provides information on the slot index and cell-specific identity. SSS may be further used to refine the CFO estimation and to detect false alarm. Specifically, if the maximum SSS correlation energy does not exceed some predetermined threshold, the current cell search results are considered to be false and the cell search needs to be continued. Once the slot index is decoded, the device can sleep until the broadcast channel, which carries system information and is located at the beginning of each downlink frame, arrives.
The two designs of cell search proposed for NB FDMA are not applicable to NB OFDMA. As for the design of cell search in the NB OFDMA, it has several issues that are described in detail as follows.
With the new MTC systems and the MTC improvements of current systems the coverage is extended. That means that many devices will operate in bad or extended coverage with much lower received signal strength levels than before. To perform the procedures associated with cell search, in particular, to estimate the time offset, the frequency offset, the cell ID, and the frame/slot ID, becomes more difficult with weak signals. This necessitates the device to accumulate multiple frames, or multiple repetitions, of sync signals to gather enough energy over time to achieve good enough detection and estimation accuracy. Accumulating multiple frames means extending the time required for synchronization and cell search, which means that the device must be active for a longer period of time, which introduces a delay and reduces the battery life.
In the design for NB OFDMA, the 8 PSCHs in a downlink frame appears non-uniformly, making it more complex to accumulate multiple PSCHs, since the offset in time between two special slots depends on the unknown slot number. In particular, 8 hypotheses are needed for accumulating 8 PSSs. Further, this irregular structure is of no particular use in the system design.
Another issue is that the cross-correlation of the SSS sequences may be poor as two combinations may have an identical portion in either SSS-1 or SSS-2. In this case, the cross-correlation of two SSS sequences equals 1+1/√71, which is only about 3 dB lower than the peak cross-correlation value 2. This makes the SSS detection accuracy vulnerable to noise, which is an undesirable property for devices operating in bad or extended coverage. To get more accurate SSS detection, accumulating multiple SSSs may be required, which introduces a delay and reduces the battery life.